52 REVIEW Continuous and high-frequency measurements in limnology: history, applications, and future challenges Pille Meinson, Agron Idrizaj, Peeter Nõges, Tiina Nõges, and Alo Laas Abstract: Over the past 15 years, an increasing number of studies in limnology have been using data from high-frequency measurements (HFM). This new technology offers scientists a chance to investigate lakes at time scales that were not possible earlier and in places where regular sampling would be complicated or even dangerous. This has allowed capturing the effects of episodic or extreme events, such as typhoons on lakes. In the present paper we review the various fields of limnology, such as monitoring, studying highly dynamic processes, lake metabolism studies, and budget calculations, where HFM has been applied, and which have benefitted most from the application. Our meta-analysis showed that more than half of the high-frequency studies from lakes were made in North America and Europe. The main field of application has been lake ecology (monitoring, lake metabolism) followed by physical limnology. Water temperature and dissolved oxygen have been the most universal and commonly measured parameters and we review the various study purposes for which these measurements have been used. Although a considerable challenge for the future, our review highlights that broadening the spatial scale of HFM would substantially broaden the applicability of these data across a spectrum of different fields. Key words: lake metabolism, temporal variability, spatial variability, extreme events, hydrodynamics. Résumé : Au cours des 15 dernières années, on observe un nombre croissant d’études en limnologie ayant utilisé des données provenant de mesures a` hautes fréquences (HFM). Cette nouvelle technologie offre aux scientifiques la possibilité d’étudier les lacs a` des échelles de temps qui étaient impossibles auparavant et dans des endroits où l’échantillonnage serait compliqué ou même dangereux. Ceci a permis de saisir les effets d’événement épisodiques ou extrêmes, tels que des typhons sur des lacs. Les auteurs passent ici en revue différents champs de la limnologie tels que le suivi, l’étude de processus hautement dynamiques, l’étude du métabolisme des lacs ainsi que le calcul des budgets, où on a appliqué la HFM et qui ont le plus bénéficié de cette application. La méta-analyse des auteurs montre que plus de la moitié des études conduites en hautes fréquences dans des lacs l’ont été en Amérique du Nord. Le principal champ d’application a concerné l’écologie des lacs (suivi, métabolisme des lacs), suivi de la limnologie physique. La température et l’oxygène dissout constituent les paramètres les plus universels et communément mesurés et les auteurs passent en revue les objectifs des diverses études pour lesquels ces mesures ont été utilisées. Bien que ceci For personal use only. constitue un défi considérable pour le futur, cette revue souligne que l’élargissement de l’échelle spatiale des HFM élargirait substantiellement l’applicabilité des données pour un ensemble de champs différents. [Traduit par la Rédaction] Mots-clés : métabolisme des lacs, variabilité temporelle, variabilité spatiale, évènements extrêmes, hydrodynamiques. 1. Introduction automatic high-frequency measurements (HFM). Even though automatic recording is vulnerable to vandalism, biofouling, Today, global environmental change and the increasing exploi- and occasional failures in the systems, and maintenance issues tation of ecosystem services by man has created complex multiple may causes gaps in time series data (Dur et al. 2007), a fast transi- stressor situations for most inland water bodies (Ormerod et al. tion to automatic HFM in environmental monitoring systems is 2010). This has obviated the need for more and better monitoring inevitable. data and is a prerequisite for understanding the often synergistic, The need for continuous observations has led to automating cumulative, and non-linear impacts of combined stressors (Brown measurements since the early times of limnology. In the first et al. 2013) and for making adequate management decisions report of the Indiana University Turkey Lake Biological Station (Hering et al. 2015). Until recently, the majority of standard lake (the first inland biological station in America), its director, monitoring programs were based on manual in situ measure- C.H. Eigenmann, mentioned, among other equipment, an “auto- ments that can be time-consuming and costly to procure and matic recording apparatus to observe seiches” (Eigenmann 1895, often lack both the necessary spatial coverage as well as an appro- p. 207). In the same year, Warren and Whipple (1895, p. 639) in- priate sampling frequency (Vos et al. 2003). The latter is especially troduced the thermophone, “a new instrument for obtaining the important for detecting the effects of hydrology or weather- temperature of a distant or inaccessible place,” and provided related episodic events, from which biological consequences can some temperature profiles measured in Lake Cochituate, Mass. range from short-term, reversible changes to those that are more Simple physical parameters, such as lake water levels and water Environ. Rev. Downloaded from www.nrcresearchpress.com by Nanjing Institute of Geography and Limnology, CAS on 01/17/17 persistent (Jennings et al. 2012). Time and reliability issues can be temperature, were among the first limnological variables for efficiently addressed by replacing manual measurements with which measurement could be automated. Parameters recorded in Received 26 May 2015. Accepted 5 October 2015. P. Meinson,* A. Idrizaj, P. Nõges, T. Nõges, and A. Laas. Centre for Limnology, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, 61117 Rannu, Tartu County, Estonia. Corresponding author: Pille Meinson (e-mail: [email protected]). *Present address: Kreutzwaldi 5, Tartu 51014, Estonia. Environ. Rev. 24: 52–62 (2016) dx.doi.org/10.1139/er-2015-0030 Published at www.nrcresearchpress.com/er on 6 October 2015. Meinson et al. 53 physical meteorology, such as air temperature, humidity, wind confront the information management and analytical challenges speed and direction, and the amount of precipitation served as posed by massive volumes of data, while Porter and Lin (2013) important background data supporting lake research (Porter et al. specifically focused on available hybrid wireless sensor network 2005). However, soon the development of electrochemistry — the technologies. Johnson et al. (2007) reviewed the chemical sensing invention of the glass electrode by Cremer (1906) and the intro- capabilities with a special focus on such chemical sensor net- duction of the concept of pH by Sørensen (1909) — gave an impe- works that can be deployed on autonomous platforms in aquatic tus to the development of various chemical sensors initially environments and then operated without significant human in- enabling automatic recording of a number of single charged ions, tervention for extended periods. Crawford et al. (2015) analyzed + + + such as H ,Na ,NH4 , proved soon as limnological key variables. the potential of using advanced sensors to investigate spatial vari- The oxygen electrode widely used in aquatic studies measuring ability in biogeochemistry and hydrology. Besides Porter et al. oxygen on a catalytic platinum surface was invented by Leland (2005), which gives a broad picture of HFM applications in envi- Clark in 1954, initially for blood gas analysis (Severinghaus and ronmental studies, some reviews give more specific insights into Astrup 1986). While chemical sensors still cannot fully compete to the use of high-frequency (HF) data in lakes. For example, with physical sensors regarding their cost or reliability, a variety Jennings et al. (2012) analyzed the potential of HFM for identifying of chemical sensing systems (e.g., for carbon dioxide, pH, and the effects of weather-related episodic events in lakes, whereas oxygen and a number of ions) are now continuously deployed in Staehr et al. (2010) reviewed the use of the diel oxygen technique aquatic environments (reviewed by Johnson et al. 2007). for studying lake metabolism. A big leap in environmental measurement and monitoring In the present paper we review various fields of limnology and technologies came with the transition from analogue to digital lake management that have benefitted or potentially may benefit technologies from the late 1990s to the early 2000s (Hilbert and from using HF measurements, in the hope of stimulating addi- López 2011). Thanks to fast technological development, the auton- tional use of this promising technology. We highlight the advan- omy of different measurement systems has increased. Using wire- tages of using HFM for various purposes compared to discrete or less systems allows retrieving data from the weather and water manual sampling, but also discuss the challenges. We put the monitoring stations in near-real time and without manually vis- main focus on HF data applications related to buoy and mooring iting the study site (Porter et al. 2005). The development of sensors systems and do not discuss other fields of HF technologies, such as has made progress by taking under consideration the protection SONAR, eco-sounding, ADCP or remote-sensing systems. We have from biofouling (Manov et al. 2004) using self-cleaning systems reviewed 154 papers using HFM in lakes and grouped the studies (e.g., wipers or pressure). After reviewing the recent develop-